Abstract
Fully coupled climate control simulations explicitly resolving physical processes over a broader range of space and time scales than those resolved in the Coupled Model Intercomparison Project 5 (CMIP5) have been carried out for periods ranging up to 150 years during the past decade. Now, we need to determine methodologies to initialize and carry out climate change simulations using these computationally demanding high-resolution coupled configurations. Towards this goal, we examine results from two distinct approaches used to initialize high-resolution present day (PD) transient simulations that approximate 1970-2015 climate change.
A 135-yr 1850 pre-industrial control (PICNTRL) simulation and five transients approximating late 20th Century climate change (present day: PD) were carried out using Energy Exascale Earth System Model version 0.1 (E3SMv0.1). It consists of the 1/4° spectral element version of the Community Atmosphere Model (CAM5-SE), the Community Land Model 4 (CLM4), and 1/10° resolution versions of the Parallel Ocean Program (POP2) and CICE4 sea ice model. An ensemble of three members were initialized from an atmospheric reanalysis-forced 0.1° POP2/CICE4 simulation run for 1948-2009 that was configured in the same framework as the fully coupled model. These initial conditions were selected to capture a spread in climate mode variability from the late 1960s to the late 1970s, prior to the strong warming of the late 20th Century. The other two PD transients were branched off the PICNTRL at years 20 and 90; the year 90 branch had reached quasi-steady state (not equilibrium). Sensitivity to initial conditions was examined by comparing mean state biases from forced POP/CICE and the PICNTRL (year 90) for the 5-year periods prior to the initial condition dates with those from the last common 5-year period (1996-2000) of the transient simulations. Strong high-latitude sea surface temperature (SST) biases led to a focus on upwelling and ocean ventilation processes in the Southern Ocean and high-latitude North Atlantic that, in turn, helped to put in context the SST distribution, mode water formation, upper ocean heat content, meridional heat and overturning circulation, and Arctic sea ice concentration depicted by the transients at the end of the 20th Century.